1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus, and especially to an image forming apparatus having a function to correct an image.
2. Description of the Related Art
In an electrophtographic image forming apparatus, in order to expose a photoreceptor, a Light-Emitting Diode (LED) may be used as a light source. LEDs can be arranged to correspond to pixels. In this case, the LEDs are disposed as an LED array, which are arranged in a main scanning direction. An image forming apparatus, which is capable of outputting a full-color image, includes a tandem type image forming apparatus having LED arrays. Here, the LED arrays are for corresponding colors of photoreceptors for developing a cyan (C) toner image, a magenta (M) toner image, a yellow (Y) toner image, and a black (K) toner image, respectively. The tandem type image forming apparatus forms a full-color image by sequentially forming and superposing images in the corresponding colors on an intermediate transfer belt or on a paper sheet. If toner image forming positions for the corresponding colors are shifted, an image having a color deviation is eventually formed.
To address this problem, in many cases, a tandem type color image forming apparatus includes a color deviation correcting function (e.g., Patent Document 1 (Japanese Unexamined Patent Publication No. 2010-140019)). FIG. 1 is a diagram illustrating an example of a color deviation correcting pattern. The color deviation correcting pattern includes lines in the corresponding colors of C, M, Y, and K, which are in parallel with the main scanning direction; and lines, which are slanted with respect to the main scanning direction by a constant angle. The image forming apparatus includes optical sensors 21 and 22 which read the color deviation correcting pattern. Here, the optical sensors 21 and 22 are separated from each other in the main scanning direction. For example, a registration deviation of the color of C in a sub-scanning direction is corrected based on a distance from a line K 11 to a line C 11, while setting the line K 11 as a reference line. A registration deviation of the color of M in the sub-scanning direction is corrected based on a distance to a line M 11, and a registration deviation of the color of Y in the sub-scanning direction is corrected based on a distance to a line Y 11. Similarly, a registration deviation of the color of C in the main scanning direction is corrected based on a distance from a line K 12 to a line C 12, while setting the line K 12 as a reference line. A registration deviation of the color of M in the main scanning direction is corrected based on a distance to a line M 12, and a registration deviation of the color of Y in the main scanning direction is corrected based on a distance to a line Y 12.
Additionally, the image forming apparatus may perform skew correction for reducing a color deviation, which is caused by tilting of the LED arrays in the main scanning direction. In the skew correction, the left and right color sensors read the correcting pattern, and a skew amount KC_Skew of the color of C with respect to the color of K is calculated, for example. The image forming apparatus performs the skew correction by shifting a timing for reading out pixel data corresponding to the skew amount KC_Skew, for a position of the pixel in the main scanning direction (by shifting the pixel, which is read out from a line memory, in the sub-scanning direction).
In the skew correction according to the related art, an amount of the color deviation is detected by the optical sensors, which are disposed at the corresponding two positions in left and right. Thus, linear skew may be corrected. However, bending skew may not be corrected. To correct the bending skew (which is skew such that a gradient is changed in the middle in the main scanning direction), another optical sensor may be added to a position other than the two positions in left and right. However, in this case, the cost may be increased.
There are many causes of the color deviation. The skew of the LED array is not necessarily the linear skew. For the case of the bending skew, if only the two optical sensors are disposed at the corresponding two positions, an amount of the bending may not be detected at a position other than the two positions. Accordingly, in the related art, the skew correction is applied while assuming that it is linearly inclined.